1. Field of the Invention
This invention relates to the use of a microbially-produced fermentation residue as a component in adhesives for wood products.
2. Description of the Prior Art
Over the last several decades renewable resources have contributed an increasing share of fuel and chemical production in developed countries. One of the largest of these contributors has been ethanol produced by fermentation and used as a gasoline additive. Commercial ethanol is produced almost exclusively by saccharification of starch (usually from corn) and subsequent fermentation of the sugars by Saccharomyces yeast. The development of fermentations based on cellulosic biomass, instead of on starch, has remained attractive because of the low cost and great abundance of cellulosic materials, either directly from biomass energy crops, or from agroforestry wastes (Lynd et al., Biotechnol. Prog. 15:777-793, 1999).
Though research on bioconversion of cellulosic materials to ethanol has largely focused on chemical or enzymatic hydrolysis of biomass with subsequent fermentation of sugars by yeast, the process is not economically viable for a variety of reasons (Lynd et al. 1999 supra). The chemical hydrolysis route suffers from a requirement for postprocessing (e.g., neutralization of the hydrolysate, the costly handling of waste products, and the removal or treatment of fermentation inhibitors formed during hydrolysis). The enzymatic route involves high costs associated with producing fungal enzyme with low inherent specific activities. A potential alternative route for cellulose bioconversion involves processes in which enzyme production, enzymatic hydrolysis and sugar fermentation occurs in a single bioreactor (Lynd et al. 1999 supra; Lynd et al., Microbiol. Molec. Biol. Revs. 66:506-577, 2002). There is little doubt that the economic viability of biomass conversion processes will ultimately depend on the marketability of co-products produced during the bioconversion process. This is implicit in the modern notion of a biorefinery that is envisioned to ultimately produce a suite of biologically-derived commercial products (Lynd et al. 1999 supra).
The ruminal cellulolytic bacterium Ruminococcus albus can ferment cellulose, some hemicelluloses (e.g., xylans and glucomannans) and pectin, to produce a mixture of ethanol, acetic acid, H2 and CO2 (Hungate, Academic Press, New York, N.Y., 1966; Pavlostathis et al., Appl. Environ. Microbiol. 54:2655-2659, 1988). A necessary prerequisite of the R. albus cellulose fermentation is adherence of the bacteria to cellulose, which is mediated by a variety of adhesins that include cellulose binding domains of cellulolytic enzymes; components of polycellulosomal organelles; pilin-like proteins and exopolysaccharide-containing glycocalyx materials (Miron et al., J. Dairy Sci. 84:1294-1309, 2001; Weimer, J. Dairy Sci. 79:1496-1502, 1996). The glycocalyx is relatively resistant to disruption by physical and chemical forces normally encountered by the organism in culture or in the rumen environment.
In unrelated work, the incorporation of natural products into chemical, industrial adhesive formulations has been explored (Loetscher, U.S. Pat. No. 1,959,433, 1934, Feigley, U.S. Pat. No. 2,868,743, 1959, Conner et al., J. Wood Chem. Technol. 6:591-613, 1986, Addition to phenol-formaldehyde (PF) resins of carbohydrates with large amounts of reducing end groups is known to result in loss of adhesive properties if the carbohydrate exceeds about 10 per cent of the weight of the PF resin (Feigley 1959 supra). By contrast, adhesive properties of PF resins are maintained upon addition of up to 30-50 per cent of the total adhesive weight of sucrose, methyl monosaccharides or sugar alcohols (Conner et al. 1986 supra).
Proteins of biological origin (e.g., blood or soybeans) were commonly used in the adhesives industry prior to the development of formaldehyde-based synthetic chemical adhesives. Neither these biological materials, nor most carbohydrates, are typically involved in adhesion in nature. However they can display adhesive properties when properly denatured, mixed with other materials, and cured under heat and pressure (Lambuth, Pizzi A., Mittal K. L., (eds) Handbook of Adhesive Technology, Marcel Dekker, New York, N.Y., pp. 259-282, 1994). The resulting mixed resins show acceptable strength under dry conditions, but often display reduced adhesive strength under wet or humid conditions (Lambuth 1994).